CAPI: Reduce #ifdef mess around CONFIG_ISDN_CAPI_MIDDLEWARE

Make the code a bit more readable be providing stub functions for the
!CONFIG_ISDN_CAPI_MIDDLEWARE case. Though a few lines are moved around,
this comes with no functional changes.

Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>